As is well known, a conventional aircraft and land vehicle braking systems are susceptible to a condition known as wheel lock during braking. This highly undesirable braking condition results from applying excessive brake torque to the wheel by the brake system that exceeds the maximum braking torque associated with the friction force between the tire and runway or road surface (herein ground/tire). The wheel lock condition normally results in a skid with, potentially, loss of vehicle directional control. Antilock brake systems (ABS) exist to try to prevent this undesirable condition and to provide optimal braking while maintaining acceptable vehicle directional controls. Prior art ABS detect undesirable braking conditions (e.g., wheel lock) and, by means of a control system and an electrohydraulic or electromechanical actuator, attempt to regulate braking to achieve the desired performance.
Brake systems, whether for aircraft or land vehicles, function by applying a retarding torque to the braked wheels of the vehicle that is in a direction opposite to the rotational direction of the wheel as the vehicle moves across a road or runway surface or any other surface (hereafter referred to as a “ground surface” regardless of actual surface type). The actual braking force that decelerates the vehicle is a function of wheel slip, i.e. the difference between the translational velocity of the vehicle (relative to the wheel axis) and the corresponding translational velocity of the wheel at the contact point with the ground surface. This braking force is directed opposite to the vehicle velocity vector and results from sliding friction between the tire and the ground surface, since the contact point is really a patch with a finite area.
An ABS system that is based upon a so-called Sliding Mode Observer (SMO) is disclosed in U.S. Pat. No. 6,890,041. In certain embodiments disclosed in the '041 patent, an SMO based estimate of net or differential wheel torque derived from the measured wheel speed is compared to a threshold differential wheel torque that may be derived as a function of a “skid signal”, also based on wheel speed only, to generate a braking or actuator control signal. The actuator control signal can be employed to rapidly and fully applying and releasing the brakes in a binary on-off manner and, as an additional option, modulating the maximum available brake hydraulic pressure or electrical current when the brakes are in the “on” state in a continuous manner. In the case of the basic on-off component of braking, the brakes are released when the estimate of differential wheel torque is less than the threshold differential wheel torque (i.e. for relatively high values of brake torque), and the brakes are applied fully when the estimate of differential wheel torque is greater than or equal to the threshold differential wheel torque. For aircraft landing gear applications, a fore-aft accelerometer mounted on the landing gear can be used to suppress nonlinear gear displacement oscillations commonly called gear walk in the direction of wheel roll.
The '041 patent describes the theory of SMO operation, the prior art, and a number of preferred embodiments in extensive detail and is incorporated by reference in its entirety herein.
In certain embodiments of the '041 patent, the threshold differential wheel torque is determined by an algorithm from past wheel speed measurements. The threshold differential wheel torque is continuously computed and compared with the estimated differential wheel torque output by the SMO. The threshold differential wheel torque is dynamically varied in accordance with the estimated differential wheel torque value and a separately calculated “skid-signal” also based on wheel angular speed only, favoring brake release as the skid signal increases and brake application as the skid signal decreases. The brakes for each wheel being controlled are released when the estimated differential wheel torque falls below the threshold differential wheel torque (on/off command signifying “off” or exhibiting “off-command”), and the brakes are applied fully (on/off command signifying “on” or exhibiting “on-command”) when the estimated differential wheel torque exceeds the threshold differential wheel torque in a limit cycle control system.
Generic electromechanical and electrohydraulic brake actuators were disclosed in the above-referenced '041 patent that operate fully “on” and fully “off” when the vehicle operator applies and releases the brakes, respectively. These on/off, i.e. switched, brake actuators typically employ solenoids that move essentially instantaneously. Switched brake actuators are typically found on general aviation and business aircraft and in automobiles and many trucks. However, it must be realized that the braking torque does not instantly follow the solenoid on/off command as the brakes themselves are electromechanical or electrohydraulic systems and changes in such systems cannot be instantaneous by the laws of nature.
The SMO-based control systems disclosed in the '041 patent were also designed to be compatible with controllers for “proportional brake actuators” that are commonly employed in large transport type aircraft, for example. Such proportional brake actuators have fairly involved transfer functions that translate an electrical input generated by a “brakes on” command into an applied brake torque applied to the wheels. As with a discrete actuator, when the pilot engages or releases the brakes using a continuous or proportional actuator the braking torque is not instantly fully “on” or fully “off”.
The determination of an appropriate threshold differential wheel torque under variable braking conditions can be complex, particularly in the case of continuous or proportional actuators employed in commercial aircraft. The algorithm for determination of a skid signal or an estimated skid signal might have to be tailored to take into account characteristics of braking systems of each particular aircraft. It would be desirable to avoid doing so and provide a universally acceptable SMO-based braking controller, particularly for use in such commercial aircraft braking systems.